In general, in an engine provided with an electronically controlled fuel injection system, in order to prevent the retardation in fuel supply in the accelerating condition of the engine, an acceleration signal detector is provided on a throttle valve. When the accelerating condition of the engine is detected by said acceleration signal detector, the fuel injection rate is increased in advance of the increase in the flow rate of intake air, so that the harmful components in the exhaust gas in the accelerating condition can be decreased and the operating performance can be improved.
The conventional acceleration signal detector used in such an electronically controlled fuel injection system has been constructed such that a switch for detecting the fully closed condition of the throttle valve is provided on the throttle valve. Such switch has been a mechanical switch adapted to be closed when the throttle valve is fully closed, whereby a reverse in the output of the switch for detecting the fully closed condition of the throttle valve from "ON" to "OFF" is detected as an acceleration, thereby effecting increased fuel injection rate, accelerated fuel injection and the like. However, when the above-described switch for detecting only the fully closed condition of the throttle valve is used, a problem is encountered in that the fuel injection system malfunctions due to chattering of the switch at the time of its operation. Particularly, in the case where a dashpot is one of the emission control parts provided in the throttle valve system, the transfer from the opening side to the closing side of the throttle valve is performed very slowly, with the result that the chattering of the switch for detecting the fully closed condition of the throttle valve occurs for a long period of time. Accordingly, this results in an increased fuel flow rate, accelerated fuel injection and the like while the throttle valve is being closed, which are actions contrary to the intended purposes.
In order to obviate the above-described disadvantages, a delay circuit having a comparatively long signal delay time of about 500 milliseconds is provided in a signal processing circuit of the switch for detecting the fully closed condition of the throttle valve. However, the provision of such a signal delay circuit presents a problem in that the delay in response during acceleration becomes large.